1. Field of the Invention
The present invention relates in general to electric motors and more particularly to brushless motors suitable for driving a blower used in an automotive air conditioner.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional brushless motor will be described with reference to FIGS. 31 and 32 of the accompanying drawings.
In FIG. 31, there is shown the conventional brushless motor "a" that is used for driving a blower fan "h" of an automotive air conditioner The brushless motor "a" comprises a housing "d" by which a drive shaft "c" is rotatably held through upperand lower bearings "b". A stator "e" is disposed about the housing "d". A cup-shaped yoke "g" is fixed to the drive shaft "c" in a manner to surround the stator "e". A plurality of ferrite magnets "f" are mounted to an inner surface of the yoke "g" keeping a small space from the stator "e". The blower fan "h" is fixed to a leading end of the drive shaft "c", as shown. The stator "e" comprises a plurality of coils "e1" which are arranged to surround the housing "d". The housing "d" is mounted on an electric parts protecting case "k" which houses therein an electric circuit board "j". The circuit board "j" comprises a drive circuit that is provided with a plurality of switching elements "i" for changing the direction of electric current flowing in the coils "e1" of the stator "e" and a motor control circuit that controls a rotation speed of the drive shaft "c" by controlling the switching elements "i". The parts protecting case "k" comprises upper and lower case parts "k1" and "k2" which are detachably coupled. The switching elements "i" are mounted on a heat sink "m" for effectively releasing heat to the outside of the case "k". The heat sink "m" is secured to the upper case part "k1" and formed with heat releasing fins "m1" which are exposed to the open air.
The detail of the heat sink "m" is shown in FIG. 32. As shown, six switching elements "i" are mounted on a rectangular lower block part of the heat sink "m", and each switching element "i" has a plurality of connecting terminals "i1" connected to given portions of the electric circuit board "j".
For assembling the switching elements "i" in the parts protecting case "k", the following assembling process has been employed. That is, before fixing the heat sink "m" to the upper case part "k1", a process for fixing the switching elements "i" to the heat sink "m" and to the electric circuit board "j" is carried out. Thus, before the heat sink "m" is fixed to the upper case part "k1", the entire weight of the heat sink "m" and that of the switching elements "i" have to be supported by only connecting terminals "i1" of the switching elements "i" that extend upward from the electric circuit board "j". Thus, to increase the mechanical strength, a so-called forming process has been applied to the connecting terminals "i1" wherein the terminals "i1" are bent to increase the durability against bending. The forming process is applied also to connecting terminals (not shown) of chemical condensers "n" that are to be connected to the wiring pattern of the electric circuit board "j". The condensers are the elements used in the drive circuit. The electric circuit board "j" has thereon a fuse (not shown) that protects the drive circuit from abnormally high current.
In the FIG. 31, denoted by references "p" are metal pins each passing through the electric circuit board "j" and having an upper end connected to the coils "e1" of the stator "e". Denoted by reference "q" is a joint terminal that passes through the electric circuit board "j" and has one end connected to the drive circuit. Denoted by references "r" are bus bars each having one end connected to a lower end of the corresponding metal pin "p" and a lower end of the joint terminal "q". Each bus bar "r" is provided with a vibration absorbing structure by which transmission of vibration from the terminal pin "p" to a first drive circuit is suppressed or at least minimized.
The electric circuit board "j" is formed with a predetermined wiring pattern. The wiring pattern is made of copper foils printed on a base "j1" made of glass or epoxy resin. A so-called etching process is used for providing such wiring pattern.
However, due to inherent construction, the above-mentioned conventional brushless motor "a" has the following drawbacks.
First, since both the drive circuit and the motor control circuit are provided on the single electric circuit board "j", the wiring pattern of the electric circuit board "j" has to be designed to bear a marked electric current that is needed by the drive circuit. In fact, the current needed by the drive circuit is greater than that needed by the motor control circuit. As a result, the copper foils of the wiring pattern have to have a thicker structure, that however causes an increased cost of the board "j" and thus that of the brushless motor "a".
Second, due to the same reason, the amount of under etching at the time of etching procedure is increased. Considering this, the copper foils of the wiring pattern of the motor control circuit have to have a large width, that makes compactness of the motor control circuit difficult.
Third, due to provision of the chemical condensers "n" mounted on the electric circuit board "j", the wiring pattern on the board "j" has to be designed taking account of a blank space on which the condensers "n" are mounted. This however makes compactness of the board "j" difficult.
Fourth, since the drive circuit and the motor control circuit on the board "j" are positioned close to each other, it becomes necessary to provide filter circuits by which a mutual interference between two circuits is prevented. This induces a bulky and costly construction of the motor "a".
Fifth, since the heat of the drive circuit is transferred to the motor control circuit through the electric circuit board "j", the parts constituting the motor control circuit have to be made of a material having a high heat resistance, which brings about a costly construction of the motor "a".